A specific feature that limits how many months in advance Clients can book online does not exist.
To do so, add repeating Blocked Time to occur every day for an extended period so that Clients won't be able to book during those times.
For example, if in January you do not want to be available for online bookings from May on, add Blocked Time beginning on May 1st. Set it to repeat every day through the end of December. As we move into the future, delete Blocked Time on a weekly or monthly basis to open up availability.